The present invention relates to a process for the activation of a catalyst comprising of metallic palladium useful for the direct oxidation of hydrogen to hydrogen peroxide. This invention particularly relates to a process for the activation by bulk oxidation of metallic palladium catalyst in order to increase its selectivity and yield in the direct oxidation of hydrogen to hydrogen peroxide and also for decreasing its hydrogen peroxide decomposition activity.
The activated catalyst manufactured by the process of this invention can be used in chemical industries for the direct of hydrogen to hydrogen peroxide.
Hydrogen peroxide is presently produced mostly by the anthraquinone process. However, this process is not environmentally clean and has several other limitations (Goor et. al, xe2x80x98Hydrogen peroxidexe2x80x99 in Ullman""s Encyclopedia of Industrial Chemistry, eds. Elver et al, p 443-466, Vol. A 13, (1989), Publisher: VCH Verlangsgeselchaft mBH, Weinheim). Demand for hydrogen peroxide is constantly increasing due to its utility in oxidising organic compounds to value added products, waste water treatment, and water disinfection. Hence, there is a great practical need for replacing the anthraquinone process by an environmentally clean and more economical process such as direct partial oxidation of hydrogen by oxygen to hydrogen peroxide with high conversion and selectivity.
U.S. Pat. No. 1,108,752 granted to Henkel et al, discloses that palladium can function as a catalyst to promote the formation of hydrogen peroxide and water from a mixture of hydrogen and oxygen. Since then, several palladium containing catalysts useful for the direct oxidation of hydrogen by oxygen to hydrogen peroxide have been disclosed by many inventors.
U.S. Pat. No. 4,832,938 of Gosser et al discloses a Pt- Pd bimetallic catalyst supported on a carbon, silica or alumina support for making hydrogen peroxide from a direct combination of hydrogen and oxygen in an aqueous reaction medium. German Patent Ger. Offen. DE 4,127,918 by Lueckoff et al discloses a supported palladium gold catalyst for the manufacture of hydrogen peroxide from hydrogen and oxygen in an aqueous medium, the catalyst containing 5-95% Au and supported on carbon. A number of platinum Group metal containing catalysts: (1) Pt-Group metal on high surface area support such as carbon, silica or alumina (U.S. Pat. No. 5,169,618); (2) Pt-Group catalyst on solid acid carrier (European Patent Application 504,741A1); (3) Pt-Group element supported on Nb- or Ta oxide (PCT Publication No. WO/9412428A1); (4) Sn-modified Pt-Group metals supported on catalysts carriers (European Patent Application FP 621,235 A1); (5) Pt-Group metal catalyst supported on hydrophillic support (U.S. Pat. No. 5,399,334); for the oxidation of hydrogen to hydrogen peroxide are known in the art.
Japanese pat. Jpn. Kokai Tokkyo Koho JP 01133909 A2 by Kyora discloses a Pt-Group metal catalyst carried on a hydrophobic support such as porous and hydrophobic Teflon support. Chuang in European Pat. EP 366419A1 discloses a Group VIII metal catalyst deposited on a hydrophobic support for the manufacture of hydrogen peroxide by reacting hydrogen with oxygen in an aqueous medium. Chuang in PCT International Application WO 9314025A1 discloses a Group VIII metal catalyst on a partially hydrophobic and partially hydrophillic support, such as Pd on fluorinated carbon, for the oxidation of hydrogen to hydrogen peroxide.
Despite extensive research reported in the prior art, direct oxidation of hydrogen to hydrogen peroxide has not been practiced so far. One reason for this could be the poor selectivity observed in the process of direct oxidation. Fu et al report a maximum selectivity of 8.7% for hydrogen peroxide in the oxidation of hydrogen by oxygen over palladium metal supported on fluorided carbon. They have also reported that the catalyst having higher hydrogen peroxide decomposition shows lower selectivity in the direct oxidation of hydrogen to hydrogen peroxide (Fu et al, Stud. Surf. Sci. Catal. Vol. 72, p 33-41, (1992)). It is therefore of great practical importance to develop a catalyst containing palladium, which shows higher selectivity for hydrogen peroxide in the direct oxidation of hydrogen by oxygen to hydrogen peroxide but has lower activity for the decomposition of hydrogen peroxide under similar conditions.
The main object of the present invention is to provide a process for the activation of a palladium containing catalyst. In order to increase the hydrogen peroxide selectivity in the direct oxidation of hydrogen by oxygen.
It is another object of the invention to develop a catalyst containing palladium, which shows higher selectivity for hydrogen peroxide in the direct oxidation of hydrogen by oxygen to hydrogen peroxide but has lower activity for the decomposition of hydrogen peroxide under similar conditions.
It is a further object of the invention to provide a process for the activation of a palladium containing catalyst such that this yield of hydrogen peroxide is enhanced.
It is yet another object of the invention to provide a process for the activation of a palladium containing catalyst so as to reduce the hydrogen peroxide decomposition activity.
These and other objects of the invention are achieved by providing a process for the activation of a catalyst comprising palladium metal useful for the direct oxidation of hydrogen to hydrogen peroxide.
Accordingly the present invention provides a process for the activation of a palladium metal containing catalyst by treating the palladium metal containing catalyst with an oxidising agent to oxidise the at least partially the the metallic palladium present in the catalyst to palladium (II) oxide (PdO).
In one embodiment of the invention, the catalyst is treated with an oxidising agent selected from the group consisting of perchloric acid, perbromic acid, periodic acid, hydrogen peroxide, nitrous oxide, oxygen, air, potassium permagnate and potassium dichromate.
In a further embodiment of the invention, the oxidising agent is selected from the group consisting of perchloric acid, hydrogen peroxide, nitrous oxide, oxygen and air.
In a further embodiment of the invention, the activation of the catalyst is carried out in liquid phase or vapor phase depending on the state of the oxidising agent.
In another embodiment of the invention, the treated catalyst may preferably be calcined at a temperature in the range of from 50xc2x0 C. to 500xc2x0 C.
In a further embodiment of the invention, the calcination of the treated catalyst is carried out at a temperature in the range of from 100xc2x0 C. to 300xc2x0 C.
In another embodiment of the invention, the catalyst may contain palladium metal with or without one or more other noble metals selected from the group consisting of platinum, ruthenium, rhodium, iridium and osmium.
In another embodiment of the invention, the catalyst may also contain palladium metal with or without one or more of promoter metals such as tin, gold and other conventional metal promoters.
In yet another embodiment of the invention, the catalyst is supported on catalyst carriers selected from the group consisting of carbon, alumina, silica-alumina, silica, zeolites, zirconia, thoria, ceria and other metal oxides or mixed metal oxides in the form of a powder, pellets, extrudes or monolith and structured metallic catalyst supports.
In yet another embodiment of the invention, surface oxidation as well as at least a partial bulk oxidation of the metallic palladium present in the catalyst to palladium (II) oxide (PdO) occur.
The production of hydrogen peroxide from the direct combination of hydrogen and oxygen is described in several patents and also in a research paper referred to above. However, the present invention is concerned with the activation of a palladium containing catalyst in order to increase the hydrogen peroxide selectivity of the catalyst in the direct oxidation of hydrogen by oxygen to hydrogen peroxide and also to decrease the hydrogen peroxide decomposition activity of the catalyst.
The palladium metal containing catalyst is activate by treating the catalyst with an oxidising agent in order to result in surface oxidation as well as at least a partial bulk oxidation of the metallic palladium present in the catalyst to palladium (II) oxide (PdO).
In the process of the invention, the activation of the palladium metal containing catalyst by its treatment with an oxidising agent involves at least a partial bulk oxidation of the metallic palladium to palladium oxide. Examples of the oxidising agent are perchloric acid, perbromic acid, periodic acid, hydrogen peroxide, nitrous oxide, oxygen or air, potassium permagnate, potassium dichromate and the like. The preferred oxidising agents are perchloric acid (HClO4), hydrogen peroxide (H2O2), nitrous oxide (N2O), oxygen and air.
The palladium metal containing catalyst may contain palladium metal with or without one or more other noble metals selected from the group consisting of platinum, ruthenium, rhodium, iridium and osmium. The catalyst may also contain palladium metal with or without one or more of promoter metals such as tin, gold and other conventional metal promoters, supported on catalyst carriers selected from the group consisting of carbon, alumina, silica-alumina, silica, zeolites, zirconia, thoria, ceria and other metal oxides or mixed metal oxides in the form of a powder, pellets, extrudes or monolith and structured metallic catalyst supports.
The treated catalyst may preferably be calcined at a temperature in the range of from 50xc2x0 C. to 500xc2x0 C., more preferably in the range of from 100xc2x0 C. to 300xc2x0 C. The activation of the catalyst is carried out in liquid phase or vapor phase depending on the state of the oxidising agent. Activation of the catalyst by a liquid oxidising agent such as perchloric acid or hydrogen peroxide may be carried out in liquid phase at a temperature below about 100xc2x0 C. After activation, the treated catalyst may be dried and calcined under vacuum or under air or inert gas such as nitrogen, helium or argon at a temperature below 500xc2x0 C., preferably at a temperature in the range of from 100xc2x0 C. to 500xc2x0 C. When the activation of the catalyst is done by a gaseous oxidising agent such as nitrous oxide or oxygen or air, the activation is done in gas phase at a temperature at or below about 500xc2x0 C. In the case of gas phase activation, it is not necessary to dry or calcine the treated catalyst.
In the process of the invention, the role of the oxidising agent is to oxidise the bulk palladium metal from the catalyst to palladium oxide at least partially and the role of the calcination of the treated catalyst is to dry and/or remove the oxidising agent remaining in the treated catalyst by decompositon.
The product obtained is an activating catalyst containing palladium oxide which is useful for the direct oxidation of hydrogen to hydrogen peroxide.
The conversion at least partially of the bulk palladium metal contained in the catalyst to palladium (II) oxide results in decreasing the hydrogen peroxide decomposition activity of the catalyst. Thereby increasing the selectivity and yield of hydrogen peroxide in the direct oxidation of hydrogen by oxygen to hydrogen peroxide is enhanced substantially,
The present invention is described with reference to the following examples illustrating the process of the invention for the activation of a palladium metal containing catalyst, useful for the direct oxidation of hydrogen by oxygen to hydrogen peroxide. The examples are illustrative and should not be construed to limit the scope of the present invention.
Conversion of H2 (%)=mole % of the hydrogen converted to all products.
Selectivity for H2O2(%)=[{conversion of H2 to H2O2(%)}/ {conversion of H2 to all products (%)}]xc3x97100
Yield of H2O2 (%)=mole (%) of H2 converted to H2O2 =[{Conversion of H2 (%)xc3x97Selectivity for H2O2(%)}] 100
Conversion of H2O2=mole % of the hydrogen peroxide decomposed to water and oxygen
The flow rate of gases is measured at 0xc2x0 C. and 1 atm. pressure. Gas Hourly Space Velocity (GHSV) is a volume of gas, measured at 0xc2x0 C. and 1 atm. pressure, passed or bubbled through unit volume of liquid reaction medium containing catalyst per hour.